ATMEGA6490V_14 Datasheet, PDF (43/392 Pages) ATMEL Corporation – High Endurance Non-volatile Memory Segments

ATmega329/3290/649/6490
Figure 10-3. MCU Start-up, RESET Extended Externally
V_CC
V_POT
RESET
V_RST
TIME-OUT
t_TOUT
INTERNAL
RESET
10.4
External Reset
An External Reset is generated by a low level on the RESET pin. Reset pulses longer than the
minimum pulse width (see “System and Reset Characteristics” on page 330) will generate a
reset, even if the clock is not running. Shorter pulses are not guaranteed to generate a reset.
When the applied signal reaches the Reset Threshold Voltage – V_RST – on its positive edge, the
delay counter starts the MCU after the Time-out period – t_TOUT – has expired.
Figure 10-4. External Reset During Operation
CC
10.5
Brown-out Detection
ATmega329/3290/649/6490 has an On-chip Brown-out Detection (BOD) circuit for monitoring
the V_CC level during operation by comparing it to a fixed trigger level. The trigger level for the
BOD can be selected by the BODLEVEL Fuses. The trigger level has a hysteresis to ensure
spike free Brown-out Detection. The hysteresis on the detection level should be interpreted as
V_BOT+ = V_BOT + V_HYST/2 and V_BOT- = V_BOT - V_HYST/2.
When the BOD is enabled, and V_CC decreases to a value below the trigger level (V_BOT- in Figure
10-5), the Brown-out Reset is immediately activated. When V_CC increases above the trigger
level (V_BOT+ in Figure 10-5), the delay counter starts the MCU after the Time-out period t_TOUT has
expired.
The BOD circuit will only detect a drop in V_CC if the voltage stays below the trigger level for lon-
ger than t_BOD given in “System and Reset Characteristics” on page 330.
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